(1) Field of the Invention
The invention relates to a seat belt retractor including two force-limiting devices acting in parallel for a motor vehicle impact protection system.
(2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A retractor of the above-mentioned type is, for example, known from WO 96/32303. The seat belt retractor described therein includes a two-piece belt shaft having a torsion bar disposed between the parts of the belt shaft. A seat belt can be wound onto one part of the belt shaft, the body of the belt shaft, while the other part of the belt shaft, also referred to as profile head, is blocked in a vehicle-fixed manner when a blocking device is activated in a vehicle-sensitive or belt-sensitive manner. In the event that the belt extraction force exceeds a predetermined belt extraction force when the second part of the belt shaft is blocked, the body of the belt shaft can rotate about its longitudinal axis in the direction of the belt extraction owing to a plastic deformation of the torsion bar. The predetermined belt extraction force to be overcome in this case is defined by the plastic deformation limit of the torsion bar. The torsion bar thus forms the first force-limiting device.
Furthermore, a second force-limiting device is provided, which acts in parallel to the torsion bar, i.e. it is also disposed between the first and second part of the belt shaft. The force-limiting characteristics can be modified using the second force limiting device independent of the torsion bar, so that, for example, degressive or progressive force-limiting characteristics can be realized. Furthermore, a short-term decrease or increase of the belt extraction force can be compensated in the initial phase of the activation of the first force-limiting device to the extent that the force-limiting characteristic of the seat belt retractor is in general more uniform and, extends as directly as possible along with the beginning of belt extraction according to a predetermined force-limiting level. The second force-limiting device can be realized by different plastically- or elastically-deformable sections, such as e.g. securing pins or a torsion sleeve which connects both parts of the belt shaft parallel to the torsion bar. The belt extraction length, during which the second force-limiting device is acting, is limited by the constructive configuration of the plastically deformable section.
Moreover, a seat belt retractor is known from DE 201 10 423 U1 wherein the second force-limiting device is formed by a metal strip which is pulled through a chicane during the force-limited belt extraction. The advantage of this solution is that the belt extraction length, during which the second force-limiting device acts, can be considerably extended by using a correspondingly long metal strip, wherein the dissipated energy and the resulting force-limiting level remain almost the same while the metal band is being pulled through the chicane. The chicane itself may be formed by an insert which is suspended on one of the parts of the belt shaft, while the metal strip is respectively held on the other part of the belt shaft in a manner which ensures tensile strength.